1. Field of the Invention
The invention relates to a device for measurement of the thickness of a first layer, comprising one or more sublayers, on a second layer of a metal sheet by X-ray fluorescence analysis, and a method of use therefor. In particular, the device comprises means defining a specimen plane for supporting the metal sheet, and further comprises means for generating and directing a beam of polychromatic primary X-rays. The beam is able to penetrate into the first and second layers for converting primary X-rays into chemical element specific fluorescent X-rays by means of absorption of the primary X-rays and re-emission of the fluorescent X-rays by the chemical element. The device further comprises means for detecting element specific fluorescent X-rays and determining an intensity thereof.
2. Description of Related Art
An X-ray device is known from U.S. Pat. No. 2,711,480. In the known device, the beam of primary X-rays is directed at an arbitrary angle on a sheet material, and relative to the beam of primary X-rays the detection means are aligned at an arbitrary angle to receive fluorescent radiation that emanates in all directions from the sheet. The known device is applicable in a method to determine the thickness of a layer of material on a chemically different base material, by measuring the attenuation of fluorescent radiation of the most abundant element comprised in the base material in passing through the layer.
However, in many cases it is not possible to use the fluorescence of the most abundant element, in particular when dealing with metal sheets comprising layers of alloys with mutually very similar compositions. Within the scope of this description, the term xe2x80x9ccladdingxe2x80x9d is used to denote the first layer, and the term xe2x80x9cclad layerxe2x80x9d is used to indicate a sublayer.
It is an object of the invention to provide a device for measurement of cladding thickness, which is capable of using fluorescence signals from elements that are present in a layer in low concentration for the thickness determination.
It is a further object of the invention to provide a device that is capable of measuring both the cladding thickness, as well as the thickness of the full metal sheet, with improved accuracy and reduced measurement time, so that the cladding thickness can be expressed as a fraction of the total metal sheet thickness.
It is a further object of the invention to provide a device for measurement of cladding thickness and metal sheet thickness with improved ease of operation, and reduced probability for operator mistakes.
It is a further object to provide a device that is suitable for use in a production environment.
It is a further object to provide a method of using the device.
According to the invention, one or more of these objects is achieved by providing a device in which the means for detection have been placed at an angle with respect to the primary beam of X-rays in dependence of the chemical element from which the fluorescent X-rays are to be detected. This achieves an improvement of the efficiency of detection, and the measurement time is reduced accordingly. Hence, a device is provided in which alloys with a low concentration of fluorescent elements can now be analysed, for determining the thickness of a cladding. In the scope of the present description, the term xe2x80x9cdetection channelxe2x80x9d will be used to denote means for detection that have been placed to selectively receive fluorescent x-rays specific for one chemical element.
The invention is based on the finding that the fluorescent radiation emanates from a metal sheet under an exit angle that is characteristic of the wavelength of the emitted fluorescent X-rays inside a host material complex. When properly excited, chemical elements are able to emit fluorescent X-rays with a spectrum of wavelengths that is characteristic for each fluorescent element. Hence, by placing the means for detection to receive fluorescent X-rays that emanate from the surface of the metal sheet under a specific exit angle, an element selective fraction of the X-rays that emanate from the metal sheet is detected. Thus the detection means receive a pre-selection of fluorescent X-rays, in favour of the fluorescent X-rays that carry the relevant information to be quantified.
In a preferred embodiment, the device comprises at least two different detection channels placed at an angle to receive fluorescent X-rays from different chemical elements. Herewith a device is provided with high flexibility in its ability to measure a variety of types of metal sheets. Additionally, the device is flexible in its ability to take into account multiple signals of fluorescent X-rays converted in different elements. The device can thus select a signal that provides the most advantageous measurement in a given operation of thickness measurement. A further advantage of the device according to this embodiment is that for each chemical element from which fluorescence is detected it is possible to measure the thickness of an additional (sub-)layer in the metal sheet.
In a preferred embodiment, the means for directing the beam of primary X-rays has been placed such that the beam of primary X-rays is directed substantially perpendicular to the specimen plane. Herewith it is achieved that primary X-rays penetrate as deep as possible into the metal sheet to be converted into fluorescent X-rays. This geometry offers the most space along the specimen plane for additional detection channels over the full azimuthal range.
In an embodiment of the invention, the means for directing a beam of primary X-rays and the means for detecting element-specific fluorescent X-rays are integrated into one measuring unit. Herewith a single measuring unit is obtained that can be moved across for instance a large metal sheet, to analyse the metal sheet in several locations. Amongst other ways to guide the motion of the integrated unit, it is particularly advantageous to provide a set of rail members for this purpose.
In an embodiment of the invention the device comprises means for pressing the measuring unit toward the specimen plane. Accordingly, during operation of this embodiment of this device, the measuring unit can be pressed against the surface of the metal sheet under investigation. This embodiment assures that the angle of incidence and exit of X-rays with respect to the surface of the metal sheet is well defined and constant. Moreover, a close contact is achieved between the metal sheet and the means for supporting the metal sheet. As a consequence, the device with pressing means is capable of performing measurements with higher accuracy than without pressing means.
Preferably, the means for pressing is pneumatic means for pressing. A high degree of control of the pressing force is obtained using a pneumatic system. Another advantage is that a quick change is made possible between a state of pressing the unit against a body and a state of release. This enables a quick succession of measurements to be made on different areas of the sheet under investigation.
In an embodiment of the invention, the device comprises means for storing an identifying label and a corresponding standard result for a plurality of standard metal sheets, and means for processing and comparing a measurement of at least one fluorescent X-ray intensity to the standard to find the identifying label of the standard metal sheet that best matches the measurement. Such a device is capable of executing metal sheet identification. In this embodiment, the appropriate materials parameters such as layer composition can now be available for extracting the correct values for the thickness of the metal sheet and/or a cladding. By preference, intensities of a plurality of detectors are compared to corresponding standard intensities. The device can then find the best matching metal sheet identification label using the combined intensities. In many practical situations the chemistry of the layers in which the fluorescent X-rays have been converted varies from metal sheet to metal sheet. With the invention a device is provided that determines the type of metal sheet in the process of a measurement of thickness. Furthermore, a correction can be applied to the determined intensity of fluorescent X-rays, in the case that the fluorescent element is present in more than one layer. A further advantage is that the device can thus be used with minimal or no operator intervention, since the device selects from the storage means the information it needs, such as alloying constituents. Otherwise it is achieved that operator errors can be discovered. In such cases the information presented on the information unit does not match the operator""s intentions. Typical operator errors are interchanging metal sheet specimens or placing the metal sheet with the wrong side facing the means for directing and detecting. In another embodiment, the device comprises an information unit for indicating the best matching standard metal sheet, or the best matching standard metal sheet as well as at least one other identification label that corresponds to a standard metal sheet that matches next best.
In a preferred embodiment, the means for supporting the metal sheet comprises a backing for converting primary X-rays into backing-specific fluorescent X-rays by means of absorption of primary X-rays and re-emission of fluorescent X-rays by the backing, wherein the backing is located such that the metal sheet is placeable between the means for generating and directing a beam of polychromatic primary X-rays and the backing. In this embodiment, a device is provided that can additionally determine the thickness of the full metal sheet by measurement of absorption of fluorescent X-rays from the backing. In certain applications it is required to express the thickness of the cladding as a fraction of the total thickness of the metal sheet. The device according to the invention can express the cladding thickness as a fraction of the total thickness of the metal sheet.
In a preferred embodiment of the device comprising a backing, the device comprises an individual detection channel for receiving fluorescent X-rays specific for the backing. This provides a device which allows for a measurement of both the cladding thickness as well as the full thickness of the metal sheet, without performing a re-alignment step when a metal sheet is inserted that is different from the previous metal sheet. In an industrial environment it is not practical to align the means for detection each time a different metal sheet is to be analysed. A further advantage of a device comprising individual detection channels is that it can simultaneously determine intensities of fluorescence emanating from the backing and of one or more chemical elements comprised in the metal sheet. Hence the device is suitable for collecting data simultaneously for determining both the full thickness of the metal sheet as well as the cladding. For each chemical element from which fluorescence is detected it is possible to measure the thickness of an additional (sub-) layer. Simultaneous detection reduces the time required for a measurement even further.
Hereafter, some embodiments of the invention will be described that offer advantages specific for measuring thicknesses in aluminum sheet. For the purpose of this application, aluminum sheet is held to comprise aluminum-alloy sheet. Currently, an important product that comprises aluminum alloy sheet material is brazing sheet. Brazing sheet is typically used in automobile radiators, air conditioner evaporators, heat exchangers, and the like. Brazing sheet is a composite material that comprises an aluminum alloy core, with on one or both sides one or more clad layers with different alloys, most often different aluminum alloys. The purpose of the cladding is to impart specific properties in the outside layer of a sheet product, such as brazing capability, corrosion resistance, erosion resistance, wear resistance, while the core alloy maintains other necessary properties such as strength.
Brazing sheet composite may be manufactured by hot rolling in which a slab of cladding material is placed to an ingot of the core material. The hot rolling process is then performed on this combination. In the final product, the core and the cladding strongly bond together, due to the fact that they are primarily of the same metal with a different content of alloying elements. Typically both core and cladding have over 80% aluminum. The process is highly delicate, and requires strict operation practices since the final sheet specification is usually rigid. Among the specifications which must be met is the cladding thickness as well as the total thickness of the brazing sheet.
Currently, these sheet specifications are measured and verified using metallographic and optical methods, involving sampling the metal sheet, preparing a metallographic mount, several steps of polishing and surface treatment, and determining the cladding and total thickness using optical microscopy. This analysis method is rather labor intensive and entails unacceptable long turn-around times of at least several hours.
To this extent, in a preferred embodiment, the backing comprises the element molybdenum. More preferably, the backing consists of essentially molybdenum. Herewith an excellent source of fluorescent X-rays is provided. The attenuation of Mo-Kxcex1, radiation is, relative to the detection accuracy, almost independent of aluminum alloy-composition in most aluminum alloys, which is advantageous in interpretation of a measurement. The attenuation of this radiation as it passes through the entire metal sheet of essentially aluminum alloy is low enough so that a high enough fraction reaches the detection means to measure an intensity in acceptable time, while at the same time the attenuation is sufficiently high to accurately determine. The typical range of thickness of aluminum alloy sheet, for example brazing sheet, that can be measured in this embodiment is between 0.07 mm and 6.35 mm. Moreover, molybdenum is sufficiently resistant against wear, and relatively inexpensive. Not least important is the fact that molybdenum does not typically exist in aluminum alloys, neither as alloying element, nor as a stray element. In a further preferred embodiment, the backing is permanently attached to a test surface of a measurement table, to form the specimen plane.
In a preferred embodiment, the device comprises at least a detection channel for receiving fluorescent X-rays specific for a chemical element of a group comprising Cu, Mn, Zn, Fe, in a metal layer comprising mainly Al. By this embodiment an intensity of fluorescent X-rays can be measured of an alloying element that is often used within an aluminum core alloy. The device can thus be used to determine the attenuation in a cladding, comprising a metal or an aluminum alloy, of fluorescent X-rays of alloying elements from a core layer. This, in turn, provides the information needed to determine a cladding thickness. Because of the alignment of the means for detecting according to the invention, the thickness of layers can now be determined using fluorescent X-rays even on alloys with a low concentration of fluorescent elements. Furthermore, a device is provided that can determine the thickness of a cladding comprising mainly the element aluminum on a further layer comprising mainly aluminum. The angle in which fluorescent X-rays propagate depends on the chemical species of the alloying element, as well as its direct surrounding within the host metal.
In a preferred embodiment, the device comprises at least different detection channels intended to receive fluorescent X-rays from each chemical element of the group comprising Cu, Mn, Zn, Fe. Herewith it is achieved that an intensity of fluorescent X-rays can be measured simultaneously of one or more of the alloying elements that are most often used within the core alloy. The device can thus be used to determine the attenuation of these fluorescent X-rays in a cladding that is located between the core and the X-ray directing and detecting means. This, in turn, provides the information needed to determine a cladding thickness. An advantage of the separate detection channels is that the device is very versatile to measure clad and cladding layer thickness on many brazing sheet products, or other coated aluminum products, wherein the alloying elements may vary from one product to another. A further advantage of the separate detection channels is that a choice can be made during operation which detector is most appropriate to utilise in the analysis. The device may comprise means for choosing which detector is most appropriate to utilise in the analysis.
In a further aspect of the invention it relates to the use of the device according to the invention for determining an intensity of element specific fluorescent X-rays emanating from a metal layer in which the fluorescent element is present in a concentration less than or equal to 20%. The device according to the invention is especially suitable for determination of the thickness of a layer or a sublayer comprising mainly a certain metal species, on a further layer comprising mainly the same metal species, and is also able to measure the thickness layers of disparate metals.
In particular, the invention further relates to the use of the device according to the invention for measurement of the thickness of a metal layer, comprising one or more sublayers, on a second layer comprising mainly aluminum alloy.
The invention further relates to the use of the device according to the invention for measurement of the thickness of a layer of aluminum alloy, comprising one or more sublayers, on a second layer comprising mainly aluminum alloy. The device as set out above is suitable for characterisation of, amongst others, aluminum sheets such as brazing sheet. The device is robust, and suitable for use in a production environment.